1. Technical Field
This invention relates to the field of AC to DC power conversion and, more particularly, to such a method and apparatus enabling an improved power factor through integrated line current control.
2. Description of Relevant Art
It is often said that the fundamental challenge of power supply design is to simultaneously realize two conflicting objectives: maximizing the electrical delivery performance of the power supply while at the same time achieving a power supply design of low cost. To this end, control circuitry for AC-DC power converters has evolved which includes a pulse width modulation circuit operated at a frequency much higher than that of the alternating current input. The pulse width modulation circuit in turn activates an electronic switching circuit for switching the rectified line current in accordance with an applied pulse width modulated signal. Such a pulse width modulation circuit is actuated by the result of a comparison of the input voltage waveform and an error signal obtained by subtracting a reference voltage from the voltage delivered to a load of the power supply.
According to the teachings of Pacholok, U.S. Pat. No. 4,472,672, an improved power factor results from forcing the input impedance of such a circuit to appear to be substantially purely resistive. Consequently, while Pacholok is capable of maximizing electrical delivery to a load, he does so at the expense of costly components such as a step-down center tap transformer for input voltage waveform sensing which make the converter uneconomical for practical application.
Retotar, U.S. Pat. No. 4,591,963, discloses a similar technique to that employed by Pacholok in the sense that the input voltage waveform is sensed and a pulse width modulator is controlled responsive to a means for combining the sensed input voltage waveform and the influences of the output voltage waveform delivered to a load upon a reference voltage. What Retotar adds to the technique disclosed by Pacholok is the application of line current sensing techniques for the purpose of constraining current in an input inductor to be in phase with the input voltage waveform. While line current sensing techniques are appropriate for control of peak current, any improvement in electrical power delivery to a load requires the application of substantially the same input line voltage waveform sensing technique as taught by Pacholok.
One problem with input line voltage waveform sensing techniques has been the inability of the control circuitry to react quickly to transients appearing in the line voltage. By the time an input voltage transient is reflected in the load voltage waveform, it is too late for control circuitry to efficiently react.
Nevertheless, the control circuitry itself has evolved to such a state that economical special purpose integrated circuits have been introduced into the marketplace for application with such voltage control techniques. These low cost integrated circuits facilitate the maximization of electrical power delivery at low cost. For example, the Unitrode Corporation UC 3842 integrated circuit includes in a single chip an oscillator, an error amplifier, a PWM latch or flip-flop and current sensing and limiting circuits at its input. At its output, a control signal may be provided for controlling, for example, the switching transistor of a current mode controlled buck regulator. From the perspective of all such known applications, there still remains a requirement to overcome the fundamental challenge presented above of achieving excellent electrical performance at low cost in an AC to DC converter and to improve transient response.